1. Field of the Invention
This invention relates to instruments for logging real-time data, such as AC (alternating current) power line parameters. More specifically, this invention relates to methods for recording data representative of an unlimited number of successive real-time samples in a limited amount of digital memory.
2. Description of Prior Art
Many parameters measured by data logging instruments vary with time. For example, common AC (alternating current) power system parameters, such as voltage, current, volt-amps, watts, and power factor, vary with time. It is often necessary to record these variations over time so that adjustments can be made to the system being measured.
One common technique for recording time-varying parameters is to convert the parameter of interest into a digital value, then periodically store this digital value in a digital memory for later examination. For example, a recorder might convert the power system voltage to a 16-bit digital value once per second, then store this digital value so voltage variations can be graphed and examined at a later time.
However, this approach consumes an ever-increasing amount of digital memory as time passes. The example above consumes 172,800 bytes of storage per day; often many weeks of data are required, so this approach is impractical when memory space is limited by cost or other considerations.
Over the last 20 years, several techniques have been developed that attempt to find a method that employs a limited amount of memory to store a potentially unlimited number of time-varying parameter samples.
One prior art technique, commonly referred to as "triggered storage", only stores parameter samples when they exceed some pre-set trigger threshold. Systems that take this approach commonly store a limited number of pre-trigger and post-trigger samples as well. One commercially available instrument that uses this technique is the Dranetz Technologies Incorporated Model 656 Power Line Analyzer. This technique succeeds when trigger levels can be selected in advance. However, when system sensitivities are unknown, it is difficult to choose proper trigger levels. If the trigger levels chosen by the user are too sensitive, memory is wasted storing data that is not required. If the trigger levels chosen are not sensitive enough, important data may be discarded. Furthermore, identical instruments that employ this technique may produce different results when measuring identical signals, due to different trigger levels.
Another prior art technique, commonly referred to as "FIFO" (first in, first out) storage, stores parameter samples sequentially in memory. When the memory is full, this technique discards the oldest sample in memory, making room for the next sample. This process continues indefinitely. One commercially available instrument that makes use of this technique is the Basic Measuring Instruments Model 4800 PowerScope. This technique does not require the user to choose triggerlevels, and identical instruments produce identical results. However, the user is required to check the instrument periodically so he can retrieve any desired data from the instrument before it is automatically discarded to make room for new data.
Another prior art technique is commonly called "LIFO" (last in first out), or "terminate when full". This technique also stores parameter samples sequentially in memory. However, when the memory is full, all new samples are discarded. As a result, once data is recorded it is never lost, but possibly valuable data points which occurred after the memory was filled will not be saved.
Another prior art technique is to maintain several memories, and fill each of them at a different rate. For example, this technique might store power system parameters once per second in one memory, once per minute in a second memory, and once per hour in a third memory. Each of these memories employs one of the three techniques described above, and most typically use the FIFO technique. A commercially available instrument that employs this technique is the Basic Measuring Instruments Model 3030 PowerProfiler. This technique preserves all data under most circumstances, and does not require trigger levels. However, it uses memory inefficiently; under many circumstances most of the memory is empty.
It is the object of the present invention to record sequential parameter samples in a fixed amount of digital memory in a way that does not require trigger levels, that always preserves some representation of every sample, and that, after initialization, always keeps memory at least half full.